SMART-1 Prepares for the End

ESA’s SMART-1 has been orbiting the Moon for 16 months, but it’s out of fuel, so its life is almost over. On September 3, 2006, it will crash into the Moon’s surface in a final act of science. If its orbit was to decay naturally, the spacecraft would impact on the far side of the Moon; however controllers have devised a strategy to help it impact on the near side, in view of Earth-based telescopes.
After sixteen months orbiting the Moon, ESA’s lunar mission is preparing for the end of its scientific exploration. On 19 June, SMART-1 mission controllers initiated a 17-day series of manoeuvres aimed at positioning the spacecraft to enhance science data return as the mission winds down.

SMART-1, Europe’s successful first Moon mission, is scheduled to end on 3 September 2006, impacting on the Moon’s surface in a disposal plan similar to that of many earlier lunar missions and almost three years to the day after its 2003 launch.

The recently started manoeuvre campaign aims to avoid having the spacecraft intersect with the Moon at a disadvantageous time from the scientific point of view, as it would have naturally about 17 August if left alone. Instead, this ‘extension’ to mission operations will provide new opportunities for low-altitude scientific observations and give optimum science returns during and after the spacecraft’s controlled impact on the Moon.

In preparation for mission end, spacecraft controllers at ESOC, ESA’s Spacecraft Operations Centre in Darmstadt, Germany, have started a series of thruster firings to give a ‘delta-velocity,’ or change in velocity, of approximately 12 metres per second. This will raise the orbit perilune (point of closest passage over the Moon) by about 90 kilometres, and will shift the impact to 3 September.

“The shift in date, time and location for Moon intersection is also optimised to favour scientific observations from Earth,” said Gerhard Schwehm, ESA’s SMART-1 Mission Manager. “Projections based on the current orbit indicated that the spacecraft, if left as is, would impact the Moon on the far side, away from ground contact and visibility. The new location is on the Moon’s near-side, at mid-southern latitudes.”

For the manoeuvre campaign, the use of the electric propulsion system (the ion engine) had to be ruled out since all Xenon propellant reserves were exhausted during the mission. The mission control team have instead developed an imaginative approach.

“The manoeuvre strategy consists of a series of reaction-wheel off-loadings combined with about three hours of intermittent thrust centred at apolune (point of furthest distance from the Moon) during the next 74 orbits,” said Octavio Camino, Spacecraft Operations Manager at ESOC.

The off-loading consists of braking a set of spinning wheels inside the spacecraft, which has the effect of transferring angular momentum from the wheels to the spacecraft and hence changing its velocity.

“We use asymmetric firing of the attitude thrusters to produce a small velocity variation aligned with the flight direction. This will change the orbit by an accumulative effect,” added Camino.

“After these manoeuvres, science activities will resume until the impact, with short interruptions for two trim manoeuvres to adjust the impact time, one around the end of July and one at the beginning of September,” he concluded.

This manoeuvre campaign and the following trim manoeuvres will make it possible to predict the exact time and location for the SMART-1 impact with more accuracy.